Process for separating an organic isocyanate

ABSTRACT

Provided is a process for separating an organic isocyanate prepared by reacting an organic amine with a stoichiometric excess of phosgene in the gas phase from the gaseous crude product obtained in the reaction, the process comprising the steps of (i) at least partially condensing the crude product stream containing at least the isocyanate, hydrogen chloride and unconverted phosgene by contacting with at least one liquid stream containing at least one quench liquid in a first separation apparatus to obtain a liquid stream containing at least some of the quench liquid and some of the isocyanate and a gas stream containing at least hydrogen chloride, evaporated quench liquid and phosgene, (ii) discharging the liquid stream obtained in step (i) via a first liquid outlet and of the gas stream obtained in (i) via a first gas conduit and (iii) at least partially condensing and/or absorbing the gas stream discharged in step (ii) through the first gas conduit, wherein that the at least partial condensation and/or absorption is effected in step (iii) by direct introduction of at least one cooling fluid, wherein the cooling fluid is introduced directly into the first gas conduit via at least one addition unit assigned to the first gas conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. Pat. No. 10,919,845, filed Sep.30, 2019 as Ser. No. 16/499,357, which is a national stage applicationunder 35 U.S.C. § 371 of PCT/EP2018/058089, filed Mar. 29, 2018, whichclaims the benefit of European Application No. 17164492.5, filed Apr. 3,2017, and Luxembourg Application No. 100164, filed Apr. 12, 2017, all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a cleaning apparatus for separating an organicisocyanate prepared by reacting an organic amine with a stoichiometricexcess of phosgene in the gas phase from the gaseous crude productobtained in the reaction, comprising a first separating unit having atleast a crude product inlet for a gaseous crude product streamcontaining at least the isocyanate, hydrogen chloride and unconvertedphosgene, a first liquid inlet for a liquid stream containing at least aquench liquid and a first liquid outlet for a liquid stream containingat least some of the quench liquid and some of the isocyanate, wherein agas conduit for conveying a gas stream containing at least hydrogenchloride, evaporated quench liquid and phosgene departs from the firstseparation unit. The invention also relates to a process for cleaningproduct streams containing organic isocyanates from a quench zone of thegas phase phosgenation. The invention further relates to a gaseousmixture for treatment in a wash column.

BACKGROUND OF THE INVENTION

There have long been general descriptions in the prior art of thepreparation of organic isocyanates, especially diisocyanates, in the gasphase, which is utilized industrially, particularly for the productionof tolylene diisocyanate, hexamethylene 1,6-diisocyanate, isophoronediisocyanate and diisocyanatodicyclohexylmethane. In all processes, agaseous crude product is obtained that comprises at least isocyanate,hydrogen chloride and unconverted phosgene (phosgene is always used inexcess), and which has to be subjected to further workup in order toobtain the desired isocyanate in pure form.

WO 2007/014936 A2, Method for producing isocyanates (in the gas phase),describes a quench zone in which the gaseous crude product is cooleddown rapidly by injecting a quench liquid. In this quench zone, thereaction mixture consisting essentially of the isocyanates, phosgene andhydrogen chloride is mixed vigorously with the liquid injected. Themixing is effected in such a way that the temperature of the reactionmixture is lowered proceeding from 200 to 570° C. down to 100 to 200°C., and the isocyanate present in the reaction mixture is transferredwholly or partly to the injected liquid droplets as a result ofcondensation, while the phosgene and the hydrogen chloride remainessentially completely in the gas phase. Possible quench liquidsmentioned are solvents, isocyanate mixtures and solvent/isocyanatemixtures. The injection of a quench liquid is mentioned for cooling ofthe reaction mixture and selective dissolution of the isocyanate formedin the solvent, with a first separation into a liquid phase and a gasphase comprising predominantly phosgene and hydrogen chloride asconstituents. The two phases are then sent to an appropriate workup.There is no discussion of further desirable optimization options forthis process step.

WO 2014/122180 A1 discloses a process for separating an isocyanateprepared by reacting a primary amine with an excess of phosgene in thegas phase from the gaseous crude product obtained in the reaction, inwhich the volume flow to the wash column is reduced by using an upstreamheat exchanger or condenser, and hence one wash column of small diameterand having a small number of separation stages is adequate. Although itis possible to reduce the size of the wash column, there can be depositsin the condenser that require cleaning and hence a shutdown of thecontinuous process. Moreover, the installation of the condenser and theoperation and maintenance thereof is associated with additional cost andinconvenience. According to the composition of the gaseous crudeproduct, its corrosive properties have to be taken into account, whichfurther increases cost and inconvenience or material demands for thecondenser.

At low condensation temperatures, there is additionally an increasedextent of unwanted condensation of phosgene out of the gas stream andhence an increase in the amount of phosgene present in the plant. Thisis undesirable for the purposes of efficient workup of the phosgene. Forexample, WO 2011/003532 A1 discusses the workup of the gas streamcontaining predominantly of phosgene and hydrogen chloride.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described for purposes of illustrationand not limitation in conjunction with the figures, wherein:

FIG. 1 shows a schematic of the cleaning apparatus known from FIG. 2 ofWO 2014/122180 A1;

FIG. 2 shows a schematic of an embodiment of the cleaning apparatus ofthe invention;

FIG. 3 shows a cross section through the first gas conduit, wherein thecooling fluid is introduced into the gas conduit via a bar equipped withnozzles on its underside;

FIG. 3A shows an arrangement of FIG. 3 in which the nozzles areintegrated into the wall of the first gas conduit;

FIG. 3B shows a variant of FIG. 3 of the addition unit without the useof nozzles;

FIG. 3C shows a further variant of the addition unit; and

FIG. 3D shows an arrangement of FIG. 3 in cross section in which thenozzles are integrated into the wall of the first gas conduit.

DETAILED DESCRIPTION OF THE INVENTION

It was therefore an object of the present invention to provide acleaning apparatus for separation of an organic isocyanate prepared byreacting an organic amine with a stoichiometric excess of phosgene inthe gas phase from the gaseous crude product obtained in the reaction,with which the formation of deposits in the wash column and inapparatuses and pipelines present between the first separation unit(quench) and the wash column can be distinctly delayed and thecondensation of phosgene out of the gas stream is very substantiallyavoided.

This object is achieved in accordance with the invention in that atleast one addition unit (11) for direct introduction of at least onecooling fluid for an at least partial condensation and/or absorption ofthe gas stream conductable through the first gas conduit (9) is assignedto the first gas conduit (9). A reduction in the volume flow rate bycondensation of the gaseous constituents is not necessarily the aimhere. What is instead desired is that phosgene remains verysubstantially in the gas phase, in order to keep the amount of phosgenepresent in the plant to a minimum. According to the mode of operation,by virtue of the addition of the cooling fluid, there can even be a(slight) increase in the volume flow. What is crucial for the inventionis instead prepurification of the gaseous product stream withoutnecessarily affecting the volume flow.

The invention thus firstly provides a cleaning apparatus (1) forseparating an organic isocyanate prepared by reacting an organic aminewith a stoichiometric excess of phosgene in the gas phase from thegaseous crude product obtained in the reaction, comprising a firstseparating unit (2) having at least

-   -   a crude product inlet (3) for a gaseous crude product stream        containing at least the isocyanate, hydrogen chloride and        unconverted phosgene,    -   a first liquid inlet (4) for a liquid stream containing at least        a quench liquid and    -   a first liquid outlet (5) for a liquid stream containing at        least some of the quench liquid and some of the isocyanate,

wherein a first gas conduit (9) for conveying a gas stream containing atleast hydrogen chloride, evaporated quench liquid and phosgene departsfrom the first separation unit (2),

characterized in that at least one addition unit (11) for directintroduction of at least one cooling fluid for an at least partialcondensation and/or absorption of the gas phase stream conductablethrough the first gas conduit (9) is assigned to the first gas conduit(9).

In the present context, cooling fluid is preferably understood to mean aliquid medium which is preferably free of compounds havingZerewitinoff-active groups that would react with isocyanate. Preferredcooling fluids are, for example, the quench liquids mentionedhereinafter. According to the preferred embodiment, the cooling fluidmay also be removed at least partly from the process, for example asliquid from a second separation unit (10). Particular preference isgiven to a quench liquid which is also used to cool down the reactionmixture exiting from the reaction zone, where the isocyanate content inthe cooling fluid is preferably ≤20% by weight, more preferably ≤10% byweight and most preferably ≤2% by weight.

In the present context, the gas stream which contains at least hydrogenchloride, evaporated quench liquid and phosgene and is conductablethrough the first gas conduit (9) is understood to mean a gas stream,which may also be an aerosol, containing a small proportion of liquiddroplets and/or solid particles. Moreover, the gas stream may alsocontain even small amounts of isocyanate that have not yet been removedcompletely in the first separation unit (2). Even though there can beincreased formation of liquid particularly downstream of the additionunit (11) essential to the invention and hence a gas phase and a liquidphase are present, this biphasic mixture conducted or conductablethrough the first gas conduit (9) is referred to hereinaftercollectively as gas stream, since the liquid components have to somedegree already passed from the first separation unit (2) into the firstgas conduit (9) in gaseous form.

In the present context, “quench zone” is understood to mean a region inwhich the gaseous crude product is cooled down rapidly by injecting aquench liquid. In this quench zone, the reaction mixture consistingessentially of the isocyanate, phosgene and hydrogen chloride is mixedvigorously with the liquid injected. The mixing is effected in such away that the temperature of the reaction mixture is lowered proceedingfrom 200 to 600° C. down to 100 to 200° C., and the isocyanate presentin the reaction mixture is transferred wholly or partly to the injectedliquid droplets as a result of condensation, while the phosgene and thehydrogen chloride remain essentially completely in the gas phase.

In the present context, the first separation unit (2) may be regarded asa quench zone. Thus, the first separation unit (2) is set up to rapidlycool and partly liquefy the gas stream conductable through the crudeproduct inlet (3) by contacting with, preferably by injecting, a quenchliquid (“quench”). Suitable first separation units (2) are described,for example, in EP 1 403 248 A1 (see especially the drawings with thecorresponding elucidations in paragraphs [0017] to [0019]), and EP 1 935875 A1 (see especially paragraphs [0015] to [0022] and [0033] to[0045]). For the conveying of the quench liquid, the separation unit (2)has a first liquid inlet (4).

Suitable quench liquids conductable through the first liquid inlet (4)are solvents, for example chlorobenzene, o-dichlorobenzene,p-dichlorobenzene, trichlorobenzenes, the corresponding chlorotoluenesor chloroxylenes, chloroethylbenzene, monochlorodiphenyl, α- orβ-naphthyl chloride, ethyl benzoate, dialkyl phthalates, diisodiethylphthalate, toluene and xylene, or mixtures thereof. Preferred solventsare chlorobenzene, o-dichlorobenzene and/or p-dichlorobenzene, andparticular preference is given to chlorobenzene or o-dichlorobenzene.This additional stream of fresh, recycled and/or processed organicsolvent preferably has a temperature of 40° C. to 150° C.

The first separation unit has a first liquid outlet (5) for a liquidstream containing at least some of the quench liquid and some of theisocyanate. In this context, a maximum proportion, preferably ≥80%, morepreferably ≥90%, more preferably ≥98% and most preferably ≥99% to≤99.95%, of the isocyanate present in the crude product stream isdesirable. In the case of the aforementioned proportions, it should benoted that the vapor pressure of isocyanate sets a natural limit for themaximum separation of the isocyanate, and economic viability has to beweighed up against the benefit of approaching this limit as closely aspossible. The gas stream obtained in this step may therefore generallystill contain small amounts of isocyanate.

It is preferable that a first liquid conduit (6) for the return of aportion of the liquid stream conductable through the first liquid outlet(5) departs from the first liquid outlet (5) and opens into the firstliquid inlet (4). If required, a pump (7) for forced conveying of thisliquid stream may be assigned to the first liquid conduit (6). It isalso possible that a quench cooler (8) for cooling of the liquid streamconductable through the second liquid conduit may be assigned to thefirst liquid conduit (6).

According to the invention, the terms “comprising” or “containing”preferably mean “consisting essentially of” and more preferably mean“consisting of”.

The phosgenation of organic amines in the gas phase is known per se andcan be effected, for example, as described in EP 0 289 840 B1, EP 1 319655 A2, EP 1 555 258 A1, EP 1 275 639 A1, EP 1 275 640 A1, EP 1 449 826A1, EP 1 754 698 B1, DE 10 359 627 A1 or DE 10 2005 042392 A1. Suitableorganic amines are, for example, aniline, pentamethylenediamine (alsoPDA hereinafter), hexamethylenediamine (also HDA hereinafter), theisomers of isophoronediamine (also IPDA hereinafter), the isomers ofxylylenediamine (also XDA hereinafter), the isomers ofdiaminodicyclohexylmethane (also H12-MDA hereinafter), the bicyclicisomers of methylenediphenyldiamine (also MDA hereinafter), and theisomers of tolylenediamine (also TDA hereinafter). Preference is givento TDA, PDA, HDA, IPDA and XDA, particular preference to TDA, PDA, HDAand IPDA, very particular preference to TDA and HDA.

The organic amine is reacted with phosgene in a continuous mode ofoperation. The reaction is preferably effected here at a temperature of200° C. to 600° C., preferably of 300° C. to 500° C., and an absolutepressure of 150 mbar to 10 bar, preferably of 1.0 bar to 3.0 bar. Themolar excess of phosgene is preferably 20% to 400% of theory. The gasstream obtained from the reaction of organic amine with phosgenecontains at least the isocyanate, hydrogen chloride and unconvertedphosgene and can be conducted through the crude product inlet (3) ascrude product stream.

In a first preferred embodiment of the invention, the first removal unit(2) and/or the first gas conduit (9) is set up such that the gas streamconductable through the first gas conduit (9) is deflected at leastonce, preferably at least twice and more preferably 2 to 5 times beforeit reaches the addition unit (11). This has the advantage that a greaterproportion of any entrained liquid droplets can be separated out. Thedeflections here can independently be effected in a vessel, whereinentry pipe and exit pipe are not disposed on opposite sides of thevessel, and/or by means of pipe bends incorporated in the first gasconduit (9). The first deflection can be effected here at an earlystage, at the transition from the first separation unit (2) into the gasconduit (9).

In a further preferred embodiment of the invention, the addition unit(11) consists of one or more nozzles set up for direct introduction ofthe cooling fluid in cocurrent or countercurrent, preferably incocurrent, or, likewise preferably, consists of multiple nozzlespositioned in the conduit shell of the first gas conduit (9), morepreferably at uniform intervals along the circumference of the first gasconduit (9). This results in the advantage that a fine mist isgenerated, which leads to further-improved agglomeration of the aerosolparticles.

More preferably, the one or more nozzles are arranged such that the mistgenerated covers the entire pipe cross section. Suitable nozzles areknown from the prior art. It is possible to use, for example, simpleperforated nozzles, but also those having internals or two-phasenozzles. Preference is given to one-phase nozzles in order to avoidadditional gas loading on the apparatuses downstream.

In a very particularly preferred embodiment of the invention, theaddition unit (11) is in a vertical pipeline section in which thegaseous crude product flows from the top downward.

In a further embodiment, it is also possible for two or more suchaddition units (11) to be arranged successively in flow direction of thegas.

As an alternative to the aforementioned preferred embodiment, it is alsopreferable when the addition unit (11) has at least one orifice andpreferably two or more orifices for generation of a fluid curtain overthe entire cross section of the first gas conduit (9), positioned in ahorizontal pipeline section in the upper region of the conduit shell ofthe first gas conduit (9), such that the gas flow crosses the lamellaeor jets of the fluid curtain. It is possible here that the orifices havea round or slot-shaped configuration, or that they are flat-jet nozzlesfor example. Also possible is the guiding of the fluid curtain through acoalescence aid mounted in the pipeline. Horizontal pipeline sections inthe context of the invention are considered to be those pipe conduitshaving a gradient of less than 60%, preferably a gradient of less than10%, more preferably a gradient of 0.01-2%. This results in theadvantage that the probability of agglomeration of the aerosol particlesis also increased further in this way.

Of the two aforementioned alternative preferred embodiments, the firstembodiment is particularly preferred since this can be particularlyeffective under some circumstances.

In a further preferred embodiment, the addition unit (11) has a coolingfluid inlet (12) for conveying the cooling fluid.

In a development of the invention, it is envisaged that the first gasconduit (9) leads to a second separation unit (10).

It is further preferable here that a second liquid conduit (13) for atleast partial recycling of a liquid stream as quench liquid departs fromthe second separation unit (10) and leads to the first separation unit(2). If required, this liquid stream can be forcibly conveyed, forexample by means of a pump (14).

The cooling fluid inlet (12) may, inter alia, originate directly from areservoir vessel for the cooling fluid. It preferably originates as abranch from the second liquid conduit (13), such that both conduits((12) and (13)) can be supplied by means of a common pump (14).

It is particularly preferable here that closed-loop control devices(15a) and (15b) for control of the liquid flow rates conductable throughthe second liquid conduit (13) to the first separation unit (2) on theone hand and through the cooling fluid inlet (12) to the addition unit(11) on the other hand are additionally assigned to the second liquidconduit (13) and to the cooling fluid inlet (12). Such closed-loopcontrol devices are sufficiently well known from the prior art. They maybe simple manual fittings by means of which a limitation of the freeconduit cross section can be established and hence the volume flow canbe limited. Preference is given to using a combination of sensor andactuator that regulates the respective flow rates in a self-containedmanner to a defined value by means of automated closed-loop control.Sensors used may, for example, be volume flowmeters such as magneticinduction flowmeters, variable area flowmeters, ultrasound flowmeters,measuring orifices, Pitot tubes or mass flowmeters such as Coriolis massflowmeters, thermal mass flowmeters or vortex mass flowmeters. Actuatorsused are, for example, motor-driven gate valves, rotary ball valves,ballcocks, piston valves or shutoff valves. This results in theadvantage that the temperatures in the first separation unit (2) and inthe addition unit (11) can be controlled via the amount of coolingfluid, and that the amount of additional cooling fluid can be adjustedto an optimally low level such that sufficient prepurification isachieved but the cost and inconvenience associated with processing orprocurement of the additional cooling fluid is minimized.

In order to obtain an additional degree of freedom in the adjustment ofthe suitable operating parameters, a heat exchanger (16) to control thetemperature of the liquid streams conductable through the conduits canbe assigned to the cooling fluid inlet (12) and/or the second liquidconduit (13). Suitable heat exchangers and their mode of operation areknown to the person skilled in the art from the prior art; it ispossible, inter alia, to use shell and tube heat exchangers or plateheat exchangers.

The second separation unit (10) may have a fourth liquid conduit (21)for addition of a liquid stream. The second separation unit (10)preferably has a second gas conduit (17) for discharge of theuncondensed gas stream, which more preferably departs in the upperportion of the second separation unit (10). A condenser (18) for atleast partial condensation of the gas stream conductable through thesecond gas conduit (17) is assigned to the second gas conduit (17). Inaddition, the condenser (18) has a third liquid conduit (20) forrecycling of the condensed liquid stream, which opens into the secondseparation unit (10).

In a further embodiment, any liquid obtained in the gas streamconductable through the first gas conduit (9) can be drained into thelower portion of the first removal unit (2), but it is preferable thatthe first gas conduit (9) runs with a gradient from the addition unit(11) to the second separation unit (10), such that gravity offerssufficient driving force to convey liquid obtained to the secondseparation unit (10) and in this way to avoid accumulations of liquid inthe region of the addition unit (11).

The second separation unit (10) is preferably a wash column with atleast one separation stage, where the wash column can be supplied withat least one solvent stream as wash liquid. Suitable wash columns (10)are described, for example, in Perry's Chemical Engineers' Handbook, 7thEdition, McGraw-Hill, chapter 14 “Gas Absorption and Gas-Liquid SystemDesign”. Preferably, the liquid stream conductable through the thirdliquid conduit (20) and/or through the fourth liquid conduit (21)contains chlorinated aromatic hydrocarbons, for example chlorobenzene,o-dichlorobenzene, p-dichlorobenzene, trichlorobenzenes, thecorresponding chlorotoluenes or chloroxylenes, chloroethylbenzene,monochlorodiphenyl, α- or β-naphthyl chloride, ethyl benzoate, dialkylphthalates, diisodiethyl phthalate, toluene and xylene. Particularlypreferred solvents are chlorobenzene and the isomers ofdichlorobenzenes; very particular preference is given to chlorobenzeneor o-dichlorobenzene. The at least one solvent stream may consist offresh solvent. However, it is also possible to use recycled, optionallyreprocessed solvent-containing streams or mixtures of the aforementionedsolvents.

In a further preferred embodiment, the fourth conduit (21) is disposedabove the separation stage in the wash column (10), and in the case ofpresence of multiple separation stages above the uppermost separationstage. When the solvent stream that is to be used is not fresh solventbut a recycled solvent-containing stream, it should be ensured that thisis essentially free of the organic isocyanate to be removed, andpreferably contains the organic isocyanate to be removed in a proportionby mass of not more than 100 ppm, based on the total mass of the liquidstream conductable through the fourth liquid conduit (21), in order toavoid the formation of breakdown products.

As an alternative to or in combination with this embodiment, the secondseparation unit (10), preferably the wash column (10), may have a fifthliquid conduit (22) for addition of a liquid stream which is fed inbelow the separation stage, and in the case of presence of multipleseparation stages below the lowermost separation stage. It may beincorporated into the separation unit itself (shown in schematic form byway of example in FIG. 2 as the fifth liquid conduit (22A)) and/or intothe corresponding drainpipe for liquids (13) (shown in schematic form byway of example in FIG. 2 as the fifth liquid conduit (22B)), and in thecase of use of a forced conveyor preferably on the suction side thereof,i.e. into the second separation unit and/or between the secondseparation unit and forced conveyor. This embodiment offers theadvantage of improved mixing of the liquid streams. When the liquidstream conductable through the fifth liquid conduit (22) is not freshsolvent but a recycled solvent-containing stream, the demands here onthe maximum isocyanate content are less strict than in the case of theliquid stream conductable through the fourth liquid conduit (21). Owingto the direct contribution of the latter stream to the vapor/liquidequilibrium with the vaporous top product stream from the wash column,which must be largely isocyanate-free, only very much smaller isocyanatecontents can be allowed in this liquid stream than in the streamconductable through the fifth liquid conduit (22). Therefore, the liquidstream conductable through the fifth liquid conduit (22) may contain upto 20% by weight of isocyanate, preferably up to 10% by weight ofisocyanate and more preferably up to 2% by weight of isocyanate, basedon the total mass of the liquid stream conductable through the fifthliquid conduit (22).

The condenser (18) has a third gas conduit (19) for conveying theuncondensed gas stream. This uncondensed gas stream consists essentiallyof hydrogen chloride gas, phosgene, further gases, for example nitrogenand carbon monoxide, and small amounts of solvent, and is preferablysent to a further workup, where solvent, excess phosgene and hydrogenchloride gas formed are generally separated from one another. Foreconomic reasons, solvent and excess phosgene are preferably sent(separately) back to the reaction. The hydrogen chloride can be sent tovarious possible uses, for example an oxychlorination of ethylene toethylene dichloride or a recycling process that recycles chlorine backinto the isocyanate process. These recycling processes include thecatalytic oxidation of hydrogen chloride, for example by the deaconprocess, the electrolysis of gaseous hydrogen chloride and theelectrolysis of an aqueous solution of hydrogen chloride (hydrochloricacid).

The operation of the addition unit (11) in the process of the inventionallows the service life before cleaning and hence an interruption incontinuous operation is required to be considerably extended compared tothe prior art. This is achieved since, under otherwise identicalpressure and temperature conditions in the inlet to the second removalunit (10), there is already more significant enrichment of entrainedorganic impurities in the liquid phase of the gas stream conductablethrough the first gas conduit (9).

The advantages of the present invention cannot be achieved by simplereduction in the condensation temperatures in the known prior artcondenser since deposits rapidly arise on the very cold surfaces of thecondenser, as a result of which it is either no longer possible tomaintain the temperature or the condenser even becomes blocked and hasto be cleaned and hence the maintenance intensity would be increased.Furthermore, in the case of operation of a condenser at lowertemperatures as described in WO 2014/122180 A1, for example, there isunwanted condensation of phosgene out of the gas stream and hence adistinct increase in the amount of phosgene present in the plant. It isimmaterial here whether the condenser is irrigated or not. What isdesirable at this point is for the phosgene to remain predominantly inthe gas phase, such that it can subsequently be purified further asdescribed in WO 2011/003532 A1 and recycled into the process.Preferably, downstream of the addition unit (11), ≥90% of the phosgeneis in the gas phase of the gas stream, more preferably ≥95% and mostpreferably >97% to ≤99.5%.

By virtue of the addition unit (11) which is essential to the invention,in operation of the cleaning apparatus (1), it is possible to achieveimproved removal of any liquid and/or solid particles present in the gasstream conductable through the first gas conduit (9). Therefore, thepresent invention further provides a process for separating an organicisocyanate prepared by reacting an organic amine with a stoichiometricexcess of phosgene in the gas phase from the gaseous crude productobtained in the reaction, comprising the steps of

-   (i) at least partial condensation of the crude product stream    containing at least the isocyanate, hydrogen chloride and    unconverted phosgene by contacting with at least one liquid stream    containing at least one quench liquid in a first separation    apparatus (2) to obtain    -   a liquid stream containing at least some of the quench liquid        and some of the isocyanate and    -   a gas stream containing at least hydrogen chloride, evaporated        quench liquid and phosgene,-   (ii) discharge of the liquid stream obtained in step (i) via a first    liquid outlet (5) and of the gas stream obtained in (i) via a first    gas conduit (9) and-   (iii) at least partial condensation and/or absorption of the gas    stream discharged in step (ii) through the first gas conduit (9),

characterized in that the at least partial condensation and/orabsorption is effected in step (iii) by direct introduction of at leastone cooling fluid, wherein the cooling fluid is introduced directly intothe first gas conduit (9) via at least one addition unit (11) assignedto the first gas conduit (9).

In a first preferred embodiment of the process of the invention, theweight ratio of the cooling fluid introduced to the gas stream conductedthrough the first gas conduit (9) is preferably 1:100 to 10:1, morepreferably 1:12 to 1:1, most preferably 1:10 to 1:2. Downstream of theaddition of the cooling fluid, via regulation of the weight ratio andoptionally the cooling fluid temperature, a temperature below that ofthe gas stream obtained in step (i) is established. At a pressure of 1.5bar(a), the temperature is preferably between 100° C. and 170° C., morepreferably between 110° C. and 150° C. and most preferably between 110and 128° C. This prevents too much phosgene from going into the liquidphase and hence an increase in the amount of phosgene present in theplant. At higher or lower pressure and depending on the cooling fluiduse or quench fluid used, the preferred temperatures may be higher orlower in accordance with the rules of thermodynamics. As a furtherpreferred embodiment, the process of the invention is conducted in sucha way that the addition unit (11) establishes a lowering of thetemperature in the gas stream by 0.5 to 50 K, preferably by 1 to 40 Kand more preferably by 3 to 30 K.

In a further preferred embodiment of the process of the invention, thecooling fluid has a temperature of ≥40 bis ≤130° C., preferably of ≥60to ≤120° C. and more preferably of ≥80 to ≤110° C. This temperaturerange leads to an effective reduction in deposits and prevents excessivewashing of phosgene out of the gas stream. In a further preferredembodiment, the cooling fluid is selected to correspond to the solventused as quench liquid in step (i).

In a further preferred embodiment, the process of the inventioncomprises the further steps of:

-   (iv) separating the gas stream from the first gas conduit (9) in a    second separation unit (10) into a liquid stream and a gas stream    and-   (v) recycling the liquid stream obtained in step (iv) as quench    liquid via a second liquid conduit (13) into the first separation    unit (2), it being possible and preferable to branch off a substream    as cooling fluid into the cooling fluid inlet (12).

It is particularly preferable here when the liquid stream recycled intostep (v) is taken from the bottoms from the second separation unit (10).Thus, in a particularly economically viable manner, the isocyanatewashed out in the second separation unit is combined with the crudeproduct in the first separation unit (2) and then they are worked uptogether with the isocyanate present therein. By means of the optionallyused heat exchanger (16), it is possible to adjust the temperature ofliquid streams recycled into step (v), which offers an additional degreeof freedom in the establishment of the suitable operating parameters.

In a further preferred embodiment of the process of the invention, thecooling fluid is chlorobenzene, o-dichlorobenzene, p-dichlorobenzene,trichlorobenzenes, the corresponding chlorotoluenes or chloroxylenes,chloroethylbenzene, monochlorodiphenyl, α- or β-naphthyl chloride, ethylbenzoate, dialkyl phthalates, diisodiethyl phthalate, toluene and/orxylene, preferably chlorobenzene, o-dichlorobenzene and/orp-dichlorobenzene and more preferably chlorobenzene oro-dichlorobenzene.

The direct addition of a cooling fluid to a gas stream from the quenchzone in the preparation of organic isocyanates leads to lower formationof deposits and therefore allows prolonged continuous operation beforethis has to be interrupted for cleaning purposes, which has a directpositive effect on space-time yield. Thus, the present invention furtherprovides for the use of a cooling fluid for optimization of the phosgeneto isocyanate mass ratio of a gas stream from the first separation unit(2), preferably from the quench zone, in an isocyanate preparation bydirect addition of the cooling fluid to a first gas conduit (9) thatconducts said gas stream. In this case, the first separation unit (2)has at least one crude product inlet (3) for a gaseous crude productstream containing at least the isocyanate, hydrogen chloride andunconverted phosgene, a first liquid inlet (4) for a liquid streamcontaining at least one quench liquid and a first liquid outlet (5) fora liquid stream containing at least a portion of the quench liquid and aportion of the isocyanate, and a first gas conduit (9) for discharge ofa gas stream containing at least hydrogen chloride, evaporated quenchliquid and phosgene. In the case of this use, preference is given tousing the aforementioned cooling fluids, quench liquids and/or solvents.

By virtue of the addition unit (11) being used in the cleaning apparatus(1) of the invention and/or in the process of the invention and/or inthe case of use of the cooling fluid in accordance with the invention,the treated gas stream has particular properties that simplifysubsequent treatment.

The present invention therefore further provides a gas stream fortreatment in at least one second separation unit (10), preferably in atleast one wash column (10), wherein the gas stream on entry into thesecond separation unit (10) includes phosgene and isocyanate in a massratio of ≥1:1 to ≤10 000:1, preferably of ≥1:1 to ≤5000:1 and morepreferably of ≥1:1 to ≤1000:1. In this context, the aforementionedgeneral and preferred configurations are likewise applicable to thesecond separation unit (10), preferably the wash column (10). The gasstream preferably corresponds to the gas stream conductable through thefirst gas conduit (9). If the gas stream, irrespective of its origin,should contain liquid fractions, the aforementioned mass ratios arebased on the contents in the gas phase.

The present invention is elucidated in detail with reference to thefigures and examples which follow, but without being restricted thereto.The figures show:

FIG. 1 shows a schematic of an embodiment of a cleaning apparatus (100)known from WO 2014/122180 A1 for separating an organic isocyanateprepared by reacting an organic amine with a stoichiometric excess ofphosgene in the gas phase from the gaseous crude product obtained in thereaction.

The cleaning apparatus (100) consists of a first separation unit (200)having a crude product inlet (300) for a gaseous crude product streamcontaining at least the isocyanate, hydrogen chloride and unconvertedphosgene. In addition, the first separation unit (200) has a firstliquid outlet (400) for the discharge of a liquid stream containing atleast some of the quench liquid and some of the isocyanate.

Also departing from the first separation unit (200) is a first gasconduit (500) for conveying a gas stream containing at least hydrogenchloride, evaporated quench liquid and phosgene, and this ends in afirst condenser (600). This condenser is set up such that at least someof the gas stream conductable through the first gas conduit (500) iscondensed. A first liquid conduit (700) for conveying the condensedliquid departs from the condenser (600).

The liquid conduit (700) ends in the first separation unit (200).Assigned to the liquid conduit (700) is a pump (800) for forcedconveying of the liquid conductable through the liquid conduit (700).The condenser (600) may additionally have a second liquid conduit (900)for feeding in a portion of the liquid stream conductable through thefirst liquid conduit (700) and/or a third liquid conduit (1000) forfeeding in a solvent stream. A second gas conduit (1100) for conveyingthe gaseous uncondensed fraction departs from the condenser and ends ina second separation unit (1200).

A fourth liquid conduit (1300) for the recycling of a liquid stream asquench liquid departs from the second separation unit (1200) and ends inthe first separation unit (200). Assigned to the fourth liquid conduit(1300) is a pump (1400) for forced conveying of the liquid streamconductable through the fourth liquid conduit (1300).

The second separation unit (1200) has a third gas conduit (1500) fordischarge of an uncondensed gas stream. A second condenser (1600) for atleast partial condensation of the gas stream conductable through thethird gas conduit (1500) is assigned to the third gas conduit (1500).The second condenser (1600) has a fourth gas conduit (1700) fordischarge of the uncondensed gas stream. In addition, the secondcondenser (1600) has a fifth liquid conduit (1800) for recycling of thecondensed liquid stream, which opens into the second separation unit(1200).

The second separation unit (1200) may additionally have a sixth liquidconduit (1900) for addition of a liquid stream, disposed in the upperportion of the second separation unit (1200). Additionally oralternatively, the second separation unit (1200) may have a seventhliquid conduit (2000) for addition of a further liquid stream, disposedin the lower portion of the second separation unit (1200).

In operation of the cleaning apparatus (100) described in FIG. 1, forseparation of an organic isocyanate prepared by reacting an organicamine with a stoichiometric excess of phosgene in the gas phase from thegaseous crude product obtained in the reaction, a gaseous crude productstream containing at least the isocyanate, hydrogen chloride andunconverted phosgene is first conducted through the crude product inlet(300) into the first separation unit (200).

In the first separation unit (200), this crude product stream is partlycondensed by adding a liquid stream containing at least one quenchliquid via the first liquid conduit (700). A liquid stream containing atleast some of the quench liquid and some of the isocyanate leaves thefirst separation unit (200) via the first liquid outlet (400).

In addition, a gas stream containing at least hydrogen chloride,evaporated quench liquid and phosgene leaves the first separation unit(200) via the first gas conduit (500) toward the first condenser (600).This gas stream is at least partly condensed in the condenser (600). Theliquid obtained is recycled into the first separation unit (200) throughthe first liquid conduit (700) with forcible conveying by means of thepump (800).

Some of the liquid conducted through the first liquid conduit (700) canbe branched off into the second liquid conduit (900) and hence fed backto the condenser (600). Alternatively or additionally, further solventcan be fed in through the third liquid conduit (1000). The uncondensedfraction in gaseous form in the condenser (600) is conveyed through thesecond gas conduit (1100) into the second separation unit (1200).

The liquid present in the second separation unit (1200) is recycled asquench liquid into the first separation unit (200) via the fourth liquidconduit (1300). This recycling is effected with forcible conveying bythe pump (1400). The gas stream uncondensed in the the second separationunit (1200) is discharged via the third gas conduit (1500) and at leastpartly condensed in the second condenser (1600). The fractionuncondensed here is discharged from the condenser (1600) via the fourthgas conduit (1700). The liquefied fraction is recycled via the liquidconduit (1800) into the second separation unit (1200).

An additional liquid stream can be supplied to the second separationunit (1200) via the optional sixth liquid conduit (1900) disposed in theupper portion of the second separation unit (1200). A further liquidstream can be supplied to the second separation unit (1200) via theoptional seventh liquid conduit (2000) disposed in the lower portion ofthe second separation unit (1200).

FIG. 2 shows a schematic embodiment of the cleaning apparatus (1) of theinvention for separating an organic isocyanate prepared by reacting anorganic amine with a stoichiometric excess of phosgene in the gas phasefrom the gaseous crude product obtained in the reaction.

The cleaning apparatus (1) consists of a first separation unit (2)having a crude product inlet (3) for a gaseous crude product streamcontaining at least the isocyanate, hydrogen chloride and unconvertedphosgene. A first liquid inlet (4) for a liquid stream containing atleast a quench liquid opens into the separation unit (2). In addition,the first separation unit (2) has a first liquid outlet (5) for a liquidstream containing at least some of the quench liquid and some of theisocyanate. A first liquid conduit (6) for the return of a portion ofthe liquid stream conductable through the first liquid outlet (5)departs from the first liquid outlet (5) and opens into the first liquidinlet (4). Assigned to the first liquid conduit (6) is a pump (7) forforced conveying of this liquid stream and a quench cooler (8) forcooling of the liquid stream conductable through the first liquidconduit.

Also departing from the first separation unit (2) is a first gas conduit(9) for conveying a gas stream containing at least hydrogen chloride,evaporated quench liquid and phosgene, and this leads to a secondseparation unit (10). To the first gas conduit (9) is assigned anaddition unit (11) for direct introduction of at least one coolingfluid. The addition unit (11) has a cooling fluid inlet (12) forconveying the cooling fluid.

A second liquid conduit (13) for the partial recycling of a liquidstream as quench liquid departs from the second separation unit (10) andends in the first separation unit (2). The cooling fluid inlet (12) may,inter alia, originate directly from a reservoir vessel for the coolingfluid. It preferably originates as a branch from the second liquidconduit (13), such that both conduits (12 and 13) can be supplied bymeans of a pump (14) assigned to the second liquid conduit (13).Closed-loop control devices (15a) and (15b) for control of the liquidflow rates through the second liquid conduit (13) to the firstseparation unit (2) on the one hand and through the cooling fluid inlet(12) to the addition unit (11) on the other hand are also assigned tothe second liquid conduit (13) and to the cooling fluid inlet (12).Downstream of the pump (14), a heat exchanger (16) for regulation of thetemperature of the liquid stream conductable through the second liquidconduit (13) is additionally assigned to the second liquid conduit (13).

The second separation unit (10) has a second gas conduit (17) fordischarge of an uncondensed gas stream. A condenser (18) for at leastpartial condensation of the gas stream conductable through second gasconduit (17) is assigned to the second gas conduit (17). The condenser(18) has a third gas conduit (19) for discharging the uncondensed gasstream. In addition, the condenser (18) has a third liquid conduit (20)for recycling of the condensed liquid stream, which opens into thesecond separation unit (10).

The second separation unit (10) additionally has a fourth liquid conduit(21) for addition of a liquid stream, disposed in the upper portion ofthe second separation unit (10). In addition, the second separation unit(10) may have a fifth liquid conduit (22A) for addition of a furtherliquid stream, disposed in the lower portion of the second separationunit (10). Alternatively, this fifth liquid conduit (22A) may open intothe second liquid conduit (13) as fifth liquid conduit (22B).

In operation of the cleaning apparatus (1) described in FIG. 2, forseparation of an organic isocyanate prepared by reacting an organicamine with a stoichiometric excess of phosgene in the gas phase from thegaseous crude product obtained in the reaction, a gaseous crude productstream containing at least the isocyanate, hydrogen chloride andunconverted phosgene is first conducted through the crude product inlet(3) into the first separation unit (2).

In the first separation unit (2), this crude product stream is partlycondensed by adding a liquid stream containing at least one quenchliquid via the first liquid inlet (4). A liquid stream containing atleast some of the quench liquid and some of the isocyanate leaves thefirst separation unit (2) via the first liquid outlet (5). A portion ofthis liquid stream is forcibly conveyed through the first liquid conduit(6) by means of the pump (7) and, having been cooled by means of thequench cooler (8), recycled into the first liquid inlet (4) and hencereused as quench liquid.

In addition, a gas stream containing at least hydrogen chloride,evaporated quench liquid and phosgene leaves the separation unit (2) viathe first gas conduit (9) to the second separation unit (10). A coolingfluid inert toward isocyanate groups is introduced directly into thefirst gas conduit (9) via the addition unit (11), as a result of whichthe gas stream is at least partly condensed and/or absorbed. The coolingfluid is conveyed through the cooling fluid inlet (12) to the additionunit (11).

The partly condensed and/or absorbed gas stream from the first gasconduit (9) is subjected to further treatment in the second separationunit (10). This treatment affords a liquid stream which is recycled atleast partly via the second liquid conduit (13) as quench liquid to thefirst separation unit (2). The recycling is effected with forcibleconveying via the pump (14) assigned to the second liquid conduit (13).The cooling fluid inlet (12) may, inter alia, originate directly from areservoir vessel for the cooling fluid. It preferably originates as abranch from the second liquid conduit (13), such that both conduits (12and 13) are supplied by means of the pump (14).

The liquid flow rates through the second liquid conduit (13) to thefirst separation unit (2) on the one hand and through the cooling fluidinlet (12) to the addition unit (11) on the other hand are controlled bythe closed-loop control units (15 a) and (15 b). The temperature of theliquid streams is controlled by the heat exchanger (16).

The fraction uncondensed in the second separation unit (10) isdischarged via the second gas conduit (17) and at least partly condensedby means of the condenser (18) assigned to the second gas conduit (17).The fraction that remains in gaseous form here is discharged from thecondenser via the third gas conduit (19) as uncondensed gas stream. Theliquefied fraction is recycled from the condenser (18) into the secondseparation unit (10) via the third liquid conduit (20). If required,additional liquid can be added to the the second separation unit (10)via the fourth liquid conduit (21) and/or the fifth liquid conduit (22A)or (22B).

The views of FIGS. 3, 3A, 3B, 3C, and 3D show a schematic of variousembodiments of the addition unit (11).

For instance, FIG. 3 shows a cross section through the first gas conduit(9), wherein the cooling fluid is introduced into the gas conduit via abar equipped with nozzles on its underside.

FIG. 3A shows an arrangement in which the nozzles are integrated intothe wall of the first gas conduit (9).

FIG. 3B shows a variant of the addition unit without the use of nozzles.The cooling fluid is added here through simple holes in the upper regionof the wall of the first gas conduit (9). A lamellar curtain is formed,which crosses the mixture flowing through the gas conduit.

FIG. 3C shows a further variant of the addition unit. This is alongitudinal section through the first gas conduit (9), wherein thecooling fluid is added via a bar with a nozzle at its end for finespraying of the cooling fluid. The gas conduit here contains a 90° pipebend and the bar is arranged such that the nozzle is downstream of thepipe bend and the cooling fluid is sprayed in cocurrent with thesubstance mixture that flows through the gas conduit.

FIG. 3D again shows an arrangement in cross section in which the nozzlesare integrated into the wall of the first gas conduit (9), this timedistributed uniformly over the pipe circumference and not all in theupper half of the pipe cross section as in FIG. 3A.

EXAMPLES Example 1 (Comparative Example)

In a pilot plant for preparation of hexamethylene diisocyanate in thegas phase, a first separation unit in which the crude product was cooleddown by means of addition of chlorobenzene was connected downstream ofthe reaction zone. This condensed 99% of the isocyanate and separated itoff as liquid crude product dissolved in chlorobenzene. The uncondensedfraction of the crude product leaving the reaction zone was cooled by afurther 10 K to 120° C. in a shell and tube heat exchanger. The heatexchanger here was irrigated here with just sufficient chlorobenzene touniformly wet its heat transfer surfaces, in order to avoid caking. Thegas stream was on the tube side of the condenser, and the heat wasremoved via a cooling water stream on the shell side of the condenser.To control the temperature, it was possible to throttle the coolingwater stream by means of a ball valve. In addition, the entrance andexit temperature on the gas side was measured continuously over theduration of the experiment. Later on, the crude product stream that hadbeen cooled down in this way was guided into a wash column, at the topof which a stream of chlorobenzene and phosgene was removed, whileorganic impurities were concentrated at the bottom and discharged as aliquid stream. In this wash column, the pressure differential betweenthe gas space below the lowermost separation stage and the gas spaceabove the uppermost separation stage was measured continuously.

In thermodynamic model calculations (ASPEN PLUS V7.2), it was shown thatabout 0.1% of the phosgene that enters the condenser was precipitated asliquid in the condenser. For this purpose, a gaseous stream consistingof 20% by weight of phosgene, 15% by weight of HCl and 65% by weight ofchlorobenzene at 1.5 bar(a) was cooled down from 130° C. to 120° C. in a“Flash2”, and the distribution of the phosgene between the liquid andgaseous output stream of the flash was examined.

Within about 12 days of continuous operation, the pressure drop in thewash column rose from originally 102 mbar to an impermissibly high valueof 150 mbar.

Example 2 (Comparative Example)

The same arrangement as in example 1 was employed. This time, however,the exit temperature of the condenser on the process side was set to 50°C. Although it was possible to operate the wash column with thiscondensation temperature for a significantly longer period than inexample 1, there was significant enrichment of phosgene in thecondensate obtained.

In thermodynamic model calculations (ASPEN PLUS V7.2), it was shown thatabout 20% of the phosgene that enters the condenser was precipitated asliquid in the condenser. For this purpose, a gaseous stream consistingof 20% by weight of phosgene, 15% by weight of HCl and 65% by weight ofchlorobenzene at 1.5 bar(a) was cooled down from 130° C. to 50° C. in a“Flash2”, and the distribution of the phosgene between the liquid andgaseous output stream of the flash was examined.

Example 3 (Inventive)

The arrangement for example 1 was modified in that the shell and tubeheat exchanger together with the first liquid outlet was replaced by asimple pipe section equipped with an addition unit for chlorobenzene.This was a simple one-phase nozzle which, as indicated in FIG. 3D, wasin the middle of the pipeline and by means of which the chlorobenzenecould be sprayed centrally into the pipeline in cocurrent. The pressurein the chlorobenzene feed to the nozzle was adjusted such that thepressure drop on exit from the nozzle led to fine atomization of theliquid. The nozzle generated a full cone with opening angle 45° incocurrent with the gas stream. The mass flow of chlorobenzene to theaddition unit was chosen such that the ratio of this and the mass flowof uncondensed crude product to the addition unit was 1:7; thetemperature of the chlorobenzene in the feed to the addition unit was95° C.; the gas stream had a temperature of 130° C. upstream of theaddition unit. Downstream of the addition unit, a temperature of 122° C.was established. After 27 days, the experiment was stopped. The pressuredrop over the addition unit and over the wash column was constant withinthis period. Even in the subsequent inspection of the apparatuses, therewere no signs of deposits.

In thermodynamic model calculations (ASPEN PLUS V7.2), it was shown thatabout 0.5% of the phosgene conducted through the gas conduit wasprecipitated in the liquid. For this purpose, a gaseous stream at 130°C. consisting of 20% by weight of phosgene, 15% by weight of HCl and 65%by weight of chlorobenzene was mixed with a liquid chlorobenzene streamat 95° C. in a mixer. Analogously to the experiment, the weight ratiowas set to 1:7 (chlorobenzene:gas stream). The mixed stream was thenseparated into a gaseous stream and a liquid stream at 1.5 bar(a) in a“Flash2” without further supply or removal of heat, and the distributionof the phosgene between these two streams was examined.

Example 4 (Inventive)

In a pilot plant for preparation of isophorone diisocyanate in the gasphase, a first separation unit in which the crude product was cooleddown by means of addition of chlorobenzene was connected downstream ofthe reaction zone. This condensed 99.2% of the isocyanate and separatedit off as liquid crude product dissolved in chlorobenzene. Theuncondensed fraction of the crude product leaving the reaction zone hada temperature of 145° C. and was discharged from the gas-liquidseparator by means of a pipeline. In this pipeline, there was anaddition unit for chlorobenzene corresponding to the arrangement inexample 3. In a departure from example 3, the mass flow of chlorobenzeneto the addition unit was this time chosen such that the ratio of thisand the mass flow of uncondensed crude product to the addition unit was1:3. Downstream of the addition unit, a temperature of 122° C. wasestablished. After 25 days, the experiment was stopped. The pressuredrop over the addition unit and over the wash column was constant withinthis period. Even in the subsequent inspection of the apparatuses, therewere no signs of deposits.

In thermodynamic model calculations (ASPEN PLUS V7.2), it was shown thatabout 1% of the phosgene conducted through the gas conduit wasprecipitated in the liquid. For this purpose, a gaseous stream at 145°C. consisting of 20% by weight of phosgene, 15% by weight of HCl and 65%by weight of chlorobenzene was mixed with a liquid chlorobenzene streamat 95° C. in a mixer. Analogously to the experiment, the weight ratiowas set to 1:3 (chlorobenzene:gas stream). The mixed stream was thenseparated into a gaseous stream and a liquid stream at 1.5 bar(a) in a“Flash2” without further supply or removal of heat, and the distributionof the phosgene between these two streams was examined.

Example 5 (Inventive)

In a pilot plant for preparation of tolylene diisocyanate in the gasphase, a first separation unit in which the crude product was cooleddown by means of addition of o-dichlorobenzene was connected downstreamof the reaction zone. This condensed 98.5% of the isocyanate andseparated it off as liquid crude product dissolved in o-dichlorobenzene.The uncondensed fraction of the crude product leaving the reaction zonehad a temperature of 170° C. and was discharged from the gas-liquidseparator by means of a pipeline. In this pipeline, there was anaddition unit for cooling fluid. Unlike in example 3, the cooling fluidthis time was withdrawn at the bottom of the second separation unit andheated to 85° C. by means of a shell and tube heat exchanger. The massflow of the cooling fluid was chosen such that the ratio of this and themass flow of uncondensed crude product to the addition unit was 1:2.Downstream of the addition unit, a temperature of 157° C. wasestablished. After 22 days, the experiment was stopped. The pressuredrop over the addition unit and over the wash column rose within thisperiod from 108 mbar to 120 mbar.

In thermodynamic model calculations (ASPEN PLUS V7.2), it was shown thatabout 1.6% of the phosgene conducted through the gas conduit wasprecipitated in the liquid. For this purpose, a gaseous stream at 170°C. consisting of 20% by weight of phosgene, 15% by weight of HCl and 65%by weight of o-dichlorobenzene was mixed with a liquid stream at 85° C.consisting of 97.5% by weight of o-dichlorobenzene and 2.5% by weight oftolylene diisocyanate in a mixer. Analogously to the experiment, theweight ratio was set to 1:2 (o-dichlorobenzene:gas stream). Thecalculated temperature of the mixture was 157° C. as in the experiment.The mixed stream was then separated into a gaseous stream and a liquidstream at 1.5 bar(a) in a “Flash2” without further supply or removal ofheat, and the distribution of the phosgene between these two streams wasexamined.

1-8. (canceled)
 9. A process for separating an organic isocyanateprepared by reacting an organic amine with a stoichiometric excess ofphosgene in the gas phase from the gaseous crude product obtained in thereaction, the process comprising the steps of (i) at least partiallycondensing the crude product stream containing at least the isocyanate,hydrogen chloride and unconverted phosgene by contacting with at leastone liquid stream containing at least one quench liquid in a firstseparation apparatus to obtain a liquid stream containing at least someof the quench liquid and some of the isocyanate and a gas streamcontaining at least hydrogen chloride, evaporated quench liquid andphosgene, (ii) discharging the liquid stream obtained in step (i) via afirst liquid outlet and of the gas stream obtained in (i) via a firstgas conduit and (iii) at least partially condensing and/or absorbing thegas stream discharged in step (ii) through the first gas conduit,wherein at least partial condensation and/or absorption is effected instep (iii) by direct introduction of at least one cooling fluid, whereinthe cooling fluid is introduced directly into the first gas conduit viaat least one addition unit assigned to the first gas conduit.
 10. Theprocess as claimed in claim 9, wherein the weight ratio of the coolingfluid introduced to the gas stream conducted through the first gasconduit is 1:100 to 10:1.
 11. The process as claimed in claim 9, whereinthe cooling fluid is selected to correspond to the solvent used asquench liquid in step (i).
 12. The process as claimed claim 9, furtherincluding the steps of (iv) separating the gas stream from the first gasconduit in a second separation unit into a liquid stream and a gasstream and (v) recycling the liquid stream obtained in step (iv) asquench liquid via a second liquid conduit into the first separationunit.
 13. The process as claimed in claim 9, wherein the cooling fluidis selected from the group consisting of chlorobenzene,o-dichlorobenzene, p-dichlorobenzene, trichlorobenzenes, thecorresponding chlorotoluenes or chloroxylenes, chloroethylbenzene,monochlorodiphenyl, α- or β-naphthyl chloride, ethyl benzoate, dialkylphthalates, diisodiethyl phthalate, toluene and/or xylene, preferablychlorobenzene, o-dichlorobenzene and p.
 14. The process as claimed inclaim 9, wherein the addition unit establishes a lowering of thetemperature in the gas stream by 0.5 to 50 K.
 15. (canceled)